mtd: nand: pxa3xx: Fix registered MTD name

In a recent commit:

  commit f455578d


  Author: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
  Date:   Mon Aug 12 14:14:53 2013 -0300

  mtd: nand: pxa3xx: Remove hardcoded mtd name

  There's no advantage in using a hardcoded name for the mtd device.
  Instead use the provided by the platform_device.

The MTD name was changed to use the one provided by the platform_device.
However, this can be problematic as some users want to set partitions
using the kernel parameter 'mtdparts', where the name is needed.

Therefore, to avoid regressions in users relying in 'mtdparts' we revert
the change and use the previous one 'pxa3xx_nand-0'.

While at it, let's put a big comment and prevent this change from happening
ever again.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>